Question

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet.

Given an arbitrary cipher text file, you need to write a C++ program to find out the value of the shift, and decrypt the cipher text to plaintext. You must not hard code the cipher text nor any shift values in your program. Test your program with a sample cipher text here (only the alphabetic letters are changed):

Projects/caesarcipher/cipher.txt ==> "

Ty ncjaezrclasj, pyncjaetzy td esp acznpdd zq eclydqzcxtyr tyqzcxletzy (cpqpccpo ez ld awltyepie) fdtyr ly lwrzctesx (nlwwpo ntaspc) ez xlvp te fycplolmwp ez lyjzyp pinpae eszdp azddpddtyr dapntlw vyzhwporp, fdflwwj cpqpccpo ez ld l vpj. Esp cpdfwe zq esp acznpdd td pyncjaepo tyqzcxletzy (ty ncjaezrclasj, cpqpccpo ez ld ntaspcepie). "

Submit the following:

1.       Source code

2.       A screen shot showing the execution of the program and the correct offset value

3.       The plaintext file

Submit each file individually. Do not zip.

Answer 1

1) Source Code

#include <iostream>
#include <string>

using namespace std;

int main() {
   string cipherText = "Ty ncjaezrclasj, pyncjaetzy td esp acznpdd zq eclydqzcxtyr tyqzcxletzy (cpqpccpo ez ld awltyepie) fdtyr ly lwrzctesx (nlwwpo ntaspc) ez xlvp te fycplolmwp ez lyjzyp pinpae eszdp azddpddtyr dapntlw vyzhwporp, fdflwwj cpqpccpo ez ld l vpj. Esp cpdfwe zq esp acznpdd td pyncjaepo tyqzcxletzy (ty ncjaezrclasj, cpqpccpo ez ld ntaspcepie).";
    int key = 11;
    char ch;
    for(int i = 0; i < cipherText.size(); ++i) {
       ch = cipherText[i];
        if(ch >= 'A' && ch <= 'Z') {
            ch = (char) ('A' + ((ch - 'A' - key + 26) % 26));
        } else if (ch >= 'a' && ch <= 'z') {
            ch = (char) ('a' + ((ch - 'a' - key + 26) % 26));
       }
        cout << ch;
    }
    cout << endl;
   return 0;
}

2) 3) In cryptography, encryption is the process of transforming information (referred to as plaintext) using an algorithm (called cipher) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information (in cryptography, referred to as ciphertext).

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